There exists on the market today several thousands of different shapes and sizes of prostheses for any given joint. Each of these implants has advantages and disadvantages with respect to many different criteria, such as, fulfillment of specific patient requirements including joint stability, function and performance, and proper sizing and fitting with the patients' joint morphology. Implants that are also tailored to specific genders or races are available (e.g., the Asian knee).
Currently, surgeons must rely on their intuition and their knowledge of the different performance characteristics and shapes of each available implant in order to try to best match it to the patient.
In particular, there is a lack of existing tools to help the surgeon accurately and efficiently determine which implant type or shape and which implant position best matches their patient's anatomy.
Computer assisted orthopedic surgery has been developed to help the surgeon plan and execute joint replacement and resurfacing operations. These systems can use models of bones taken from pre-operative or intra-operative image data (for example CT's, MRI, Ultrasound, fluoroscopy, . . . ), or they can use image free techniques, such as, Bone Morphing where a model is deformed to points acquired on a surface of a bone. Alternatively, a hybrid approach can be used in which a pre-established model can be warped or morphed to acquired image data to aid in the model building process. Several publications exist in the literature describing the various techniques and just a few of them are listed:                Lavallee—System for determining the position of a knee prosthesis US20050101966; and        Fluete—Restoring of three-dimensional surfaces by using statistical models U.S. Pat. No. 7,227,981.        
Each of the above listed references is hereby incorporated by reference in its entirety.
Different planning systems also exist to aid the surgeon in determining the optimal position and size of the prosthesis. These systems generally work by measuring different dimensions of the bone in different areas and comparing these with the corresponding dimensions of a particular size of one type of implant from a range of sizes of the same implant type.
A disadvantage of the existing systems for joint implant surgery is that, although they help the surgeon in selecting what size of prosthesis to install, they do not provide any significant assistance in determining which type of implant and/or which implant position provides the best fit to the patents anatomy. They do not help the surgeon identify how the subtleness of a particular prosthesis shape or prosthesis position fits the detailed and specific anatomy of the patient. This can be important for restoring optimal joint function and kinematics. In particular, femoral component rotation in knee arthroplasty is a key issue that can affect femoro-tibial and femoro-patellar kinematics. However, current systems primarily use only basic references (axes) and/or knee gap measurements to determine femoral rotation. They do not help the surgeon in an intuitive way to visualize how a particular change in the prosthesis position/rotation impacts the joint surfaces.